linux-kernel - Re: [PATCH v2 2/2] mm: fix missing cache flush for all tail pages of THP

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Message-ID: <5D9B52E1-A74B-4964-AACF-ADB91536C4C0@nvidia.com>
Date:   Mon, 24 Jan 2022 14:22:33 -0500
From:   Zi Yan <ziy@...dia.com>
To:     David Rientjes <rientjes@...gle.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, Lars Persson <lars.persson@...s.com>
Subject: Re: [PATCH v2 2/2] mm: fix missing cache flush for all tail pages of THP

On 24 Jan 2022, at 13:11, David Rientjes wrote:

> On Mon, 24 Jan 2022, Muchun Song wrote:
>
>> The D-cache maintenance inside move_to_new_page() only consider one page,
>> there is still D-cache maintenance issue for tail pages of THP. Fix this
>> by not using flush_dcache_folio() since it is not backportable.
>>
>
> The mention of being backportable suggests that we should backport this,
> likely to 4.14+.  So should it be marked as stable?

Hmm, after more digging, I am not sure if the bug exists. For THP migration,
flush_cache_range() is used in remove_migration_pmd(). The flush_dcache_page()
was added by Lars Persson (cc’d) to solve the data corruption on MIPS[1],
but THP migration is only enabled on x86_64, PPC_BOOK3S_64, and ARM64.

To make code more consistent, I guess flush_cache_range() in remove_migration_pmd()
can be removed, since it is superseded by the flush_dcache_page() below.

The Fixes can be dropped. Let me know if I miss anything.

>
> That aside, should there be a follow-up patch that converts to using
> flush_dcache_folio()?

Are you suggesting to convert just this code or the entire move_to_new_page()
to use folio? The latter might be more desirable, since the code will be
more consistent.


[1] https://lore.kernel.org/all/20190315083502.11849-1-larper@axis.com/T/#u

>
>> Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> ---
>> Changes in v2:
>>  - Using a for loop instead of the folio variant for backportable.
>>
>>  mm/migrate.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index c9296d63878d..c418e8d92b9c 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -933,9 +933,12 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>>  		if (!PageMappingFlags(page))
>>  			page->mapping = NULL;
>>
>> -		if (likely(!is_zone_device_page(newpage)))
>> -			flush_dcache_page(newpage);
>> +		if (likely(!is_zone_device_page(newpage))) {
>> +			int i, nr = compound_nr(newpage);
>>
>> +			for (i = 0; i < nr; i++)
>> +				flush_dcache_page(newpage + i);
>> +		}
>>  	}
>>  out:
>>  	return rc;
>> -- 
>> 2.11.0
>>
>>
>>

--
Best Regards,
Yan, Zi


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